The present invention relates generally to plastic bags and, more particularly, relates to a method and apparatus for sealing a drawtape to a hem of a thermoplastic film structure.
Sealable polymeric packages, such as trash bags, are a common household item. The bags come to the consumer in the form of a roll of interconnected bags or as pre-separated bags housed in a dispensing box. When the bags are provided in the form of a roll, one end of the bag, the bottom, is thermally sealed closed and connected to its neighboring bag along a perforated line; the other end of the bag, the open mouth end, is attached to its neighboring bag solely along another perforated line. When the bags are pre-separated, neighboring bags are generally overlapped or interleaved in such a manner that removal of one bag from the dispensing box draws the neighboring bag toward an opening in the box.
In order to close a typical polymeric bag after it has been filled by the consumer, the bag body adjacent the open mouth end of the bag is gathered and tied into a knot or secured using a separate tie member supplied by the vendor of the bags. Tie members typically include paper coated flexible wires, rubber bands, or strips of plastic having a locking mechanism to provide a means to pull tight and securely fasten the neck of the bag. The need for separate tie members, however, adds an additional cost factor for the manufacturer, and ultimately, the consumer. In addition, separate tie members are easily lost and hence can be a nuisance for the consumer. Polymeric packages having integral closure systems overcome these problems. Such integral closure systems can be in the form of tie members, adhesives and the like.
One particularly advantageous closure system is a drawtape or drawstring that is integral to the bag body. Bags of this type are typically in the form of a pair of pliable thermoplastic body panels joined to each other along a pair of opposing sides and a bottom bridging the opposing sides. The bag may be opened along a mouth end formed opposite the bottom. The body panels form a hem along the mouth end of the bag, and the hem houses a pliable thermoplastic drawtape. One or more drawtape holes located within the hem expose the drawtape allowing it to be pulled through the holes to close the bag and to be used as a handle.
Examples of such drawtape bags are disclosed in U.S. Pat. Nos. 4,597,750, 4,624,654, and 4,854,983. In the manufacture of drawtape bags, the body panels and the hem are formed from a continuous sheet of thermoplastic film. The sheet is folded in half to form the opposing panels and the free ends of the respective panels are folded over to form respective hems. Drawtapes are then inserted into the respective hems, and the hems are sealed to the respective panels to maintain the drawtapes within the respective hems. Next, the ends of each drawtape are presealed to the respective hem at the areas where the side seals of the bags are to be made to insure that the drawtapes will not pull loose from the side seals. In one presealing technique, the hem-carrying sheet is stopped and heated seal bars are pressed against the preseal area while the sheet is stopped. In another presealing technique, the hem-carrying sheet is continuously moved over a cylindrical drum having a heated seal bar mounted flush with its cylindrical surface. The sheet is pressed against the cylindrical surface by at least one rubber nip roll and is heat sealed in the area of the seal bar. Although such techniques typically generate adequate preseals, the present inventors have discovered a technique for generating preseals without moving parts, which improves speed efficiency, mechanical reliability, and process consistency.
These and other objects are realized by a method and apparatus for sealing a drawtape to a hem of a thermoplastic film structure. The thermoplastic film structure includes a panel, the hem, and the drawtape. The hem is formed along one end of the panel, and the drawtape is housed within the hem. In the method, the film structure is conveyed to a stationary heat sealer that includes a stationary heated block and a hot air blower spaced from the heated block. The film structure is momentarily stopped at the stationary heat sealer with hem disposed between the heated block and the hot air blower. While the film structure is stopped, the hot air blower blows heated air onto the hem to force the hem against the heated block, thereby sealing the drawtape to the hem.
The above summary of the present invention is not intended to represent each embodiment, or every aspect of the present invention. This is the purpose of the figures and detailed description which follow.